Such loudspeakers are known, for example, from international application WO97/09842, and counterpart U.S. application Ser. No. 08/707,012, filed Sep. 3, 1996, both to New Transducers Ltd. In general, such speakers include a resonant bending wave acoustic radiator, e.g. in the form of a plate, and a transducer mounted on the plate to convert electrical signals into mechanical vibrations. The transducer excites the resonant bending wave modes in the plate, which then emits sound to create an acoustic output.
The properties of the acoustic radiator may be chosen to distribute the resonant bending wave modes substantially evenly in frequency. In other words, the properties or parameters, e.g. size, thickness, shape, material, etc., of the acoustic radiator may be chosen to smooth peaks in the frequency response caused by “bunching” or clustering of the modes. The resultant distribution of resonant bending wave modes may thus be such that there are substantially minimal clusterings and disparities of spacing.
In particular, the properties of the acoustic radiator may be chosen to distribute the lower frequency resonant bending wave modes substantially evenly in frequency. The number of resonant bending wave modes is less at lower frequencies than at higher frequencies and thus the distribution of the lower frequency resonant bending wave modes is particularly important. The lower frequency resonant bending wave modes are preferably the ten to twenty lowest frequency resonant bending wave modes of the acoustic radiator. The resonant bending wave modes associated with each conceptual axis of the acoustic radiator may be arranged to be interleaved in frequency. Each conceptual axis has an associated lowest fundamental frequency (conceptual frequency) and higher modes at spaced frequencies. By interleaving the modes associated with each axis, the substantially even distribution may be achieved. There may be two conceptual axes and the axes may be symmetry axes. For example, for a rectangular acoustic radiator, the axes may be a short and a long axis parallel to a short and a long side of the acoustic radiator respectively. For an elliptical acoustic radiator, the axes may correspond to the major and minor axis of the ellipse. The axes may be orthogonal.
The transducer location may be chosen to couple substantially evenly to the resonant bending wave modes. In particular, the transducer location may be chosen to couple substantially evenly to lower frequency resonant bending wave modes. In other words, the transducer may be mounted at a location spaced away from nodes (or dead spots) of as many lower frequency resonant modes as possible. Thus the transducer may be at a location where the number of vibrationally active resonance anti-nodes is relatively high and conversely the number of resonance nodes is relatively low. Any such location may be used, but the most convenient locations (for a rectangular panel) are the near-central locations between 38% to 62% along each of the length and width axes of the panel, but off-central. Specific locations found suitable are at 3/7, 4/9 or 5/13 of the distance along the axes; a different ratio for the length axis and the width axis is preferred.
A particularly preferred kind of exciter for use with bending wave loudspeakers is the inertial exciter, an example of which is shown attached to a panel form member 15 in FIG. 1. The exciter 14 comprises an electromagnetic motor made up of a magnet assembly and a voice coil assembly. The magnet assembly comprises a magnet 20, a pole piece 22 and a magnet cup 24 such that the magnet 20 is sandwiched between and attached to both the pole piece 22 and the magnet cup 24.
The voice coil assembly comprises a voice coil 26 wound on a former 27 which is attached to a coupler ring 28 which in turn is mounted on a mounting surface 30 of the panel-form member 15. The magnet assembly 20,22,24 is mounted on the voice coil assembly by means of a suspension 32 attached between the voice coil former 27 and the magnet cup 24.
Through audio connections (leads) 34, the exciter 14 receives electrical signals which are fed to voice coil 26. In accordance with well-known electromagnetic principles, these signals result in a force being exerted on the magnet assembly, with a reaction force being exerted on the voice coil, coupler ring and finally the panel 15. As a result of the higher mass (inertia) of the magnet assembly, it is the panel 15 that moves and, in combination with the preferential positioning mentioned above, generates sound.
The present inventors have identified two problems with known methods of mounting the magnet assembly. Firstly, when installed on a non-horizontal panel as shown in FIG. 1, the exciter tends to “creep”, i.e. twist on its suspension under the effect of the weight, W, of the magnet assembly acting through its centre of mass, M. Secondly, the exciter may exhibit rocking modes which degrade power handling, shorten life, and increase distortion. In particular, leakage of energy into rocking modes may impair the power delivery at the lowest frequencies.
Further issues surround the mounting of the exciter as a whole. As is known, it may be advantageous to attach an exciter to a bending wave, panel-form loudspeaker by means of adhesive. However, should an exciter attached in this manner develop a fault, it will be necessary to break the adhesive joint and remove adhesive residue from the surface of the loudspeaker panel before a replacement exciter can be attached by means of a new adhesive bond.